Satellites provide communication coverage over a specified surface area on Earth. As discussed herein, a cell is a geographical coverage area on the surface of the Earth and a spot beam is a radiation pattern of an antenna that illuminates a cell. A surface spectral density (Hertz per square kilometer (“Hz/km2”) within the coverage area is typically increased by increasing the number of radiated spot beams to partition the coverage area into multiple cells and reusing the available spectrum many times. For instance, dividing an area previously covered by one broad beam into 19 cells covered by 19 narrow spot beams and splitting the frequency spectrum into four equal parts (and reusing the spectrum in smaller cells) results in a surface spectral density that is increased by a factor of 19/4 or nearly five-times. To provide broad and uniform coverage with a high surface spectral density, the satellite or telecommunications platform accordingly may use a plurality of antennas such that each antenna is configured to provide similar communication coverage (e.g., a spot beam) to a cell. It is also common practice to create multiple beams from a single antenna by using more than one duplex feed for each antenna. Phased array and beamforming techniques are also well-known.
Generally, antennas with identical dimensions and properties are chosen to reduce design variation. However, the variation of distances between a platform and various cells, resulting in part from the curvature of the Earth, leads to differently sized cells from identical antennas. For instance, cells directly below a platform are relatively smaller compared to cells at the edges of a specified coverage area. If the same amount of spectrum is used in each cell then the surface spectral density is greater for the smaller cells below the platform compared to the surface spectral density of the larger cells at the outer edges of coverage. Differences in surface spectral density between cells can result in service disruptions and/or service degradations or a difference in the perceived quality of service as a user moves between cells or as the cells move past the users.
In geostationary Earth Orbit (“GEO”) satellite systems, the effects of cell size differences are generally not severe or noticeable. For instance, at the geostationary orbit height of 35,786 km above the surface, Earth subtends an angle of only 20°. This means that except at the extreme edges of satellite coverage where the surface curves away from the satellite, cells within a coverage area are generally uniform. However, for satellites at lower altitudes, the differences between cell sizes are more pronounced. For instance, Earth subtends an angle of about 160° relative to a high altitude platform operating between 17 km and 22 km above the surface. This greater angle causes significant differences in cell areas covered by the same antennas. Such differences in cell size can affect the quality of service (“QoS”) because coverage in the cells located on the perimeter of the coverage area is subject to higher path loss and lower surface spectral density, thereby resulting in lower user bandwidth.
GEO satellites are stationary relative to a point on Earth but satellites in other orbits move relative to fixed points on Earth. For the latter systems, therefore even stationary user terminals will be served by different spot beams over time, i.e., the cells move with the telecommunications platform. If different cells use the same amount of frequency spectrum but have different sizes, the available bandwidth for each user terminal changes, which may cause service disruptions.